1. Field of the Invention
The present invention relates to continuous polymerization processes and more particularly relates to an improved system for controlling the temperature of the reactor during such processes.
2. Background of the Invention
Polymerization refers to the process of linking small molecules (monomers) to make larger molecules (polymers). Polymers, for example polyolefins such as polyethylene, when combined with other ingredients may be manufactured into commercially useful plastics, rubbers, fibers and coatings. There are a number of processes for achieving polymerization, each designed for various conditions and end products.
A typical system for commercially producing a diversity of polymers is generally shown in FIG. 1. The chemical reactions to produce the polymer take place in a chemical reactor 10. The chemical reactor may take the form of a fluidized bed, a gas-phase reactor, a stirred tank, or a slurry reactor. The reactor is continuously fed with a gaseous monomer through a closed-loop gaseous supply line 11. The inlet 21 for the gaseous stream is at a point in the lower region of the reactor usually at the very bottom of the reactor to insure adequate uniformity of the gaseous stream passing upwardly through the reactor. A catalyst is continuously added to the reactor to activate the polymer producing reaction. Normally the rate of catalyst injection is used to control the rate of polymer production. The reacted polymer product is removed from the reactor and the portion of the gaseous stream which did not react exits the reactor and becomes the recycled stream. New monomer gas is added to the recycled stream to make up for the removed polymer.
The polymer forming reaction within the reactor is exothermic, making it necessary to maintain in some fashion the temperature of the gas stream inside the reactor. An essentially constant temperature in the reactor is necessary to produce polymer product having consistent properties. The temperature within the reactor is basically dependent on three factors: 1) the rate of catalyst injection into the reactor which controls the rate of polymerization and the attendant rate of heat generation; 2) the temperature of the gas recycle stream and 3) the volume of the recycle stream passing through the reactor. A common method of heat removal employed in conventional continuous polymerization processes is by compression and cooling of the recycled gas stream at a point external to the reactor. The temperature of the reactor is usually controlled by compressing the recycled stream in a compressor 12 and then passing the stream through a heat exchanger 13 where the heat of the chemical reaction is removed from the recycled stream before it is returned to the reactor.
The heat exchanger 13 typically includes a coolant supply line 14 connected to the exchanger which includes a pump 15 for advancing the coolant through the exchanger, a fresh coolant inlet 16, a recirculated coolant discharge 17 and a valve 18 for adjusting the proportion of fresh coolant to recirculated coolant. The heat from the recycled gas stream is transferred to the coolant within the heat exchanger and the cooled recycled stream returns to the reactor.
A common temperature control strategy for polymerization reactors consists of cascading the reactor temperature to the coolant temperature in a 2-level control scheme. A master controller 19 monitors the reactor temperature and transmits a signal to a slave controller 20 when the temperature within the reactor increases above the master controller's set point. The signal transmitted to the slave controller is a new lower coolant temperature set point for the slave controller. In conventional systems, the slave controller monitors the temperature of the recycled stream at the inlet 21 of the reactor with respect to its own set point. In response to a new set point from the master controller, the slave controller increases the flow of fresh coolant by opening the control valve of the heat exchanger coolant line. The amount that the control valve is opened by the slave controller is dependent on the slave controller's set point, provided by the master controller, and the temperature of the recycled gas being monitored at the reactor inlet 21.
However, there is significant dead time between the time the fresh coolant rate changes and the time the reactor gas inlet temperature responds. This dead time is the time required for the coolant to travel from the fresh coolant inlet to the heat exchanger plus the time taken for the recycle stream of gas exiting the heat exchanger to reach the reactor inlet. As a result, the slave controller will continue to change the fresh coolant flow rate until the reactor inlet temperature responds. Often, the slave controller overshoots during the dead time before the controller knows it has gone too far which results in overcooling of the recycled stream during the dead time. This makes it necessary to keep the slave controller from responding too quickly. On the other hand, if the controller acts too slowly, reactor temperature can move far away from its set point whereby the reactor overheats before the controller responds. Accordingly, there is a need for an improved temperature control method for a continuous polymerization process which provides excellent responsiveness in maintaining reactor temperature near a set point during load changes of production.